Phase shift detector (PSD) is utilized to detect the phase difference caused by signal shift, and it can be applied to different fields, like optics, chemistry and communication fields. In optics, pulsed light is sent into nonlinear crystal, and crystal correlation coefficient can be obtained by the measured phase difference from the PSD. In chemistry, the phase difference measured by the PSD can be utilized to analyze the characteristic of chemical element. And the PSD also can be applied to clock and data recovery (COR), wireless receiver and phase shift modulator in communication field. The phase difference is utilized to perform matter analysis or data operation in each application, so how to precisely detect the signal phase for improving whole system accuracy is an important issue in recent years.
With reference to FIG. 7, it is a block diagram of a conventional phase frequency detection system. The conventional phase frequency detection system which includes a phase frequency detector (PFD), a charge pump, a loop filter, a voltage-controlled oscillator (VCO) and a frequency divider (DIV) can capture input signal phase by phase locked loop (PLL). However, the charge pump will stop charging and discharging because the PLL is stable and the PFD is not stable yet in PLL actual operation, so cause the VCO stop operating and generate an undetected phase zone which is called dead zone, and the range of the dead zone will affect whole system accuracy.